The ability to determine the RMS value of an unknown, e.g., AC signal with a high degree of accuracy is of critical importance in many environments. Presently, AC voltmeters provide accuracy to about 0.1% (1000 parts per million.) When higher accuracy is required, transfer standards are used. Transfer standards are used to measure the RMS value of an unknown AC signal by determining the difference between the RMS value of the unknown AC signal and a preset, accurately measured DC equivalent. While the use of transfer standards provides accuracy in the 100 parts per million (ppm) range, it has a number of disadvantages. First, the cost of the transfer standards test equipment required to achieve this accuracy is higher than desired. Secondly, and more importantly, the time required to make a transfer standards measurement having an accuracy in the 100 ppm range normally requires several (e.g. 5) minutes. As a result, the use of a transfer standards approach to determining the RMS value of a signal is both costly and time consumming. Thus, a need exists for an inexpensive measuring system for producing a signal that accurately represents the RMS value of an unknown AC signal.
Therefore, it is an object of this invention to provide a new and improved RMS conversion method and apparatus.
It is another object of this invention to provide a highly accurate RMS converter system.
It is a further object of this invention to provide a highly accurate RMS conversion method and apparatus that relatively rapidly converts an AC input signal into a DC signal having a magnitude proportional to the RMS value of the AC signal.
It is a still further object of this invention to provide a new and improved RMS converter system that relatively rapidly and quickly accurately determines the RMS value of an unknown AC signal.
While AC voltmeters and transfer standards have been utilized in the past to determine the RMS value of an unknown signal, other, less accurate and substantially less expensive, devices have been developed for converting an unknown AC signal into a DC signal having a magnitude that is equal to the RMS value of the AC signal. One form of such systems applies to AC signal to be converted to a first heating element such as a thermal resistor. The heat produced is thermally coupled to a suitable heat sensor, such as a transistor, which is connected in a differential circuit with a similar heat sensor. The differential output is utilized to control the DC power applied to a second heating element thermally coupled to the second heat sensor. At balance, the DC feedback voltage applied to the second heating element is equal to the RMS value of the unknown AC signal applied to the first heating element. Devices of this type having a conversion accuracy of 0.5% have been produced. Such a device is disclosed in U.S. patent application Ser. No. 842,972, filed Oct. 17, 1977 by Roy W. Chapel Jr. And I. Macit Gurol and entitled "Thermally Isolated Monolithic Semiconductor Die." While an accuracy of 0.5% (5000 ppm) is inadequate in many environments, these RMS converters have the advantage that they can be relatively inexpensively produced. Thus, it would be desirable to utilize such devices in more accurate RMS converter systems. Therefore, it is yet another object of this invention to provide a new and improved RMS converter system that utilizes relatively inexpensive thermal RMS converters.